Cleanup session code, docs fixes for Shopify

[davidmuzi]

• consolidates access_token storage in all services
• makes shop_domain public
• docs fixes

[calebkleveter]

@davidmuzi Are you ready for this to be merged, or do you want to make more changes first?

[davidmuzi]

Good to merge!

I was running through starting a new app from scratch and was running into this fatal error:

public var container: Container {
        guard let c = _container else {
            fatalError("If you encounter this error, you are holding on to a client after de-initializing your Application. This is usually a bad idea.")
        }
        return c
    }

It looks like there where 2 request objects in public func fetchToken(from request: Request) throws -> EventLoopFuture<String> that where in scope, and the wrong one was being used. Oddly, the app I created along side the work I was doing in Imperial works fine 🤷‍♂️ Either way, it obviously makes sense that I renamed the variables to not cause this confusion.